1. Field of the Invention
This invention relates to integrated circuit (IC) chips packaged in plastic encapsulating material. More particularly, this invention relates to precision linear IC chips in plastic packages arranged to provide enhanced manufacturing yields and increased reliability in operation.
2. Description of the Prior Art
IC chips are packaged in various ways depending upon the type of chip and its intended application. One of the least expensive packaging techniques is that of completely encapsulating the chip with plastic in a mechanized high-volume molding operation. Such plastic packaging is used extensively for many types of chips. However, it has not found significant usage for precision linear IC chips, because such chips could not with known plastic packaging techniques provide a high manufacturing yield or perform reliably in the field. Thus, conventionally, precision linear chips have been packaged in hermetically sealed packages, e.g. formed of ceramic, at a higher unit cost than plastic packaging. Thus, attention has been directed for some time to solving the problem of packaging precision linear ICs in plastic.
It has been proposed in the art to apply a so-called overcoat to the surface of an IC chip prior to its being packaged in plastic, for the purpose of reducing penetration of moisture to the chip surface and thereby to prevent corrosion. However, experience with such an overcoat application has shown serious problems such as breakage of wires bonded to the contact pads.
Considerable research has recently been directed to analysis of forces produced by a molded plastic package on the surface of the contained IC chip, to determine such matters as the resulting displacement of aluminum interconnects with changes in temperature of the plastic encapsulation or the piezo-resistivity effects on diffused or implanted resistors. Some published studies, for example, have concerned the use of test structures to make quantitative measurements of compressive stresses and qualitative observations of shear stress effects. None of this work on force analysis has, however, led to insights into ways of significantly improving the manufacturing yields and performance of plastic encapsulated precision linear IC chips.
Accordingly, there has existed a long felt need to achieve the capability of assembling precision linear devices in plastic packages, with both high graded yields and reliable performance in use. It is a principal purpose of this invention to solve that problem.